SNP MSP Alex Neil said he was "very very confident" at least another bid would be made for the Edinburgh-based bank.
And he and Scottish Liberal Democrat leader Tavish Scott and independent MSP Margo MacDonald accused HBOS chiefs of failing in their duty to examine all the possible options for the bank's future.
HBOS is already the subject of a proposed 12 billion merger with Lloyds TSB.
Today Mr Neil said: "I'm confident within the next week or so that at least one rival bid will become public knowledge."
It emerged at the weekend that talks had taken place between Scottish Secretary Jim Murphy and leading businessman Jim Spowart about a possible alternative bid for HBOS.
And Mr Neil explained: "Very often any potential rival bidders wait to see the prospectus of the initial bidder then they finalise the bid they intend to make.
"I am very very confident there will be at least one other rival bid and that that will become public knowledge in due course."
Mr Neil spoke out at a joint press conference with Mr Scott and Lothians MSP Ms MacDonald.
There Mr Scott argued: "The non-executive directors of HBOS must explore all the options on behalf of shareholders, I think that's their duty here to explore all the options that are available.
"I find it extraordinary that those non-executive directors haven't been in touch with any other party to explore those options. They need to do that very quickly indeed."
Ms MacDonald said: "It's quite likely that the fiduciary duty incumbent on the people who run the bank is being ignored, because there's a duty to inform shareholders of all the options, of all of the information that will be relevant when they come to vote."
And she added: "Information is being withheld from shareholders."
Similarly Mr Neil, a member of Holyrood's Finance Committee, said: "The HBOS directors have a fiduciary duty, a statutory duty, to ensure they get the best deal for their shareholders.
"They have not explored any other options other than the Lloyds TSB option. That is a dereliction of duty in our view.
"They should have been in touch with Jim Spowart and anyone else who is talking about a rival bid to get the appropriate information and discuss seriously with people what the rival bid might entail, so that they can then make a judgment on the basis of the information about the two or more bids what the best deal is for the shareholder.
"And they have singularly failed to do that."
Mr Neil claimed he had "very very serious concerns" about the Lloyds TSB merger.
He continued: "We don't think it's the right way forward for the bank, for Scotland, for shareholders, for consumers or in the interests of society more generally."
He also claimed there was an "arrogant presumption" made by senior figures in both Lloyds TSB and HBOS that the deal would go through and added: "They seem to forget that the shareholders have not yet, and I don't believe they will, approve this merger."
He also revealed he had written to the Takeover Panel, the independent body which regulates takeover bids and other mergers, regarding his concerns about the deal.
Mr Neil said: "By any measure the shareholders are being treated with contempt, the people who work for HBOS and Lloyds TSB are being treated with contempt, the tax payer is being treated with contempt and there is strong case for the Takeover Panel to do what it should it do in terms of its duty and call a halt to this process until we get clear answers."
Ms MacDonald, a Lothians MSP, added: "There are any number of technical questions and good business practice questions that still have to be asked.
"That's why I get so angry with the assumption that there is only one game in town and the normal check and balances to such a merger or takeover are being waived."
Meanwhile Mr Scott said "All we're asking for is for another bid to be allowed the fair and transparent level playing field which it should be allowed."
But Labour's Holyrood leader Iain Gray said: "For over a month now Alex Neil of the SNP has been going on about an alternative bid.
"It is time he said who they are and they came to the table. It is irresponsible to raise hopes if there is in fact nothing concrete there.
"It is outrageous to use thousands of people's jobs as political pawns in what looks increasingly like political posturing."
He added: "Labour's priority is working closely with Unite, the bank workers union, to protect as many jobs as possible.
"Labour believes any genuine alternative bid deserves an equal and fair hearing and the details of such a bid should be fully examined and considered but it must be brought forward for that to happen."
And Scottish Conservative leader Annabel Goldie said: "Unless Tavish Scott, Alex Neil, and Margo MacDonald are bringing forward their own bid for HBOS there is no other bid."
Miss Goldie added: "The only alternative to the Lloyds TSB takeover, failing another bid, is effectively nationalisation of HBOS – not an outcome the bank, the taxpayer or anyone else wants to see."